Polyester is a polymer obtained through polycondensation of a polybasic acid and a polyalcohol. Since various types of polymers for different purposes or applications can be synthesized by combining polybasic acids and polyalcohols, these polymers can be used in various fields, such as those producing films, containers and fibers.
As a polyester superior in heat resistance and crystallinity, there has been reported a vanillic polyester as a polymer of an aromatic hydroxycarboxylic acid, especially as a polymer of a vanillic acid which is useful as a biomass resource (Patent documents 1 and 2). Conventionally, titanium catalysts have been used as polymerization catalysts for obtaining a vanillic polyester through polymerization. There has been a problem that a vanillic polyester obtained using a titanium catalyst is already colored in brown, which creates a hindrance to coloring under various intended purposes.
Meanwhile, non-patent document 1 discloses a method for producing a vanillic polyester, using an antimony catalyst. In general, when a polyester is used for the aforementioned purposes such as containers, there cannot be achieved a sufficient strength unless the molecular weight of the polyester is sufficiently high, thus requiring a method for achieving a high molecular weight. However, a vanillic acid polyester with a high molecular weight cannot be obtained by the method described in the non-patent document 1. In fact, a vanillic acid polyester with a weight average molecular weight of about 5,000 is at the highest (Non-patent document 1).